and other cells. Surrounding the grey matter is the white matter of the spinal cord, which is composed of the ascending and descending neural tracts. These tracts (collections of nerve fibers) connect the brain with the rest of the body. The spinal cord plays the role of a huge cable, which exchanges innumerable messages with the CNS.
There are two main functions of the spinal cord. Firstly, it carries sensory impulses coming from the lower parts of the body to the brain; and motor impulses originating from the brain to all over the body. Secondly, it performs some simple reflexes that do not involve the brain. Simple reflexes involve a sensory nerve, a motor nerve, and the association neurons of the grey matter of the spinal cord.
Reflex Action A reflex is an involuntary action that occurs very quickly after its specific kind of stimulation. The reflex action takes place automatically without conscious decision of the brain. Reflex actions are inherited in our nervous system through evolutionary processes, for example, the eye-blinking reflex. Whenever any object suddenly comes near our eyes, our eyelids blink.
Reflexes serve to protect the organism from potential threats and preserve life. Though several reflex actions are performed by our nervous system, the familiar reflexes are the knee jerk, pupil constriction, pulling away from very hot or cold objects, breathing and stretching. Most reflex actions are carried out by the spinal cord and do not involve the brain. The Endocrine System The endocrine glands play a crucial role in our development and behaviour.
They secrete specific chemical substances, called hormones, which control some of our behaviours. These glands are called ductless glands or endocrine glands, because they do not have any duct (unlike other glands) to send their secretions to specific places. Hormones are circulated by the bloodstream. The endocrine glands form the endocrine system of the body.
This system works in conjunction with different parts of the nervous system. The whole system is thus known as neuroendocrine system. Figure . shows the major endocrine glands of the body.
Some hormones are secreted at a